The use of certain metal ions as antibacterial and antimicrobial agents is known in the art. For example, in the medical community, it is known to incorporate metal ions such as silver ions, copper ions and zinc ions into and/or onto medical devices and appliances such as orthopedic pins, plates and implants, wound dressings, urinary catheters, and the like to guard against infections. Typically, these metal ions, and particularly silver ions because of their unusually effective bioactivity at low concentrations, have been incorporated into and/or onto the medical devices both as inorganic and/organic soluble salts that are intended to leach from the devices during use to prevent and treat microbial infections. While metal ions in the form of soluble salts are effective for controlling microbial infections, they do not provide prolonged protection due to the gradual loss during use. Accordingly, additional metal ions must be reapplied at frequent intervals to maintain the effectiveness of the antimicrobial activity, and this is often very difficult, particularly where an in-dwelling or implanted medical device is involved.
As a result, there have been attempts to prepare medical devices that involve the use of low solubility compounds and/or complexes of metal ions, particularly silver ions, which leach from the devices during use at a relatively slow rate. For example, U.S. Pat. No. 2,785,153 suggested using a colloidal silver protein for this purpose. Such compounds typically would be formulated as creams and would be applied as coatings. While the release of silver ions from such formulations is quite slow, the coatings from such formulations have witnessed little practical use due to adhesion, abrasion resistance and shelf life problems.
The use of silver metal coatings for antimicrobial purposes also has been suggested. However, it is generally accepted that such coatings do not provide the required level of efficacy, since the diffusion of silver ions from the metallic coating is negligible. One attempt at improving the antimicrobial utility of silver metal coatings is disclosed in U.S. Pat. No. 6,238,686. In that patent, it is suggested to deposit the metal coating by vapor deposition techniques to produce atomic disorder in the coating such that a sustained release of metal ions sufficient to produce an antimicrobial effect is achieved. Among the numerous patent publications suggesting the use of silver metal and/or silver ions in medical devices and appliances there may be listed U.S. Pat. No. 6,264,936, U.S. Published Patent Application 2002/0073891, and PCT International Publication WO 92/13491.
The use of metal ions as an antimicrobial agent is also known outside of the medical arts. For example, U.S. Pat. No. 6,383,273 discloses a process for producing a catalyst and/or adsorbent composition that contains a colloidal metal oxide or colloidal metalloid oxide binder, a support, such as a polymer, carbon, a cellulosic fiber or a metal oxide, which can absorb and/or adsorb an antimicrobial agent, and an antimicrobial agent, such as silver nitrate or copper nitrate. The antimicrobial compositions disclosed in that patent are said to be useful for reducing or eliminating the amount of bioactive agent or contaminant in a variety of environments, such as water and air.
The use of metal ions to produce antimicrobial glass compositions also is known. For example, in Japanese Publication 10-158037, it is disclosed to prepare an antimicrobial glass by exchanging alkali metal ions in the glass with silver ions. This is accomplished by dipping a glass substrate into a bath of molten salt that contains substantial silver. The bath temperature higher than the melting point of the silver salt, but lower than the glass transition temperature of substrate. In the examples of this publication, the temperature at which the fused salt bath was heated ranged from 240° C., when using silver nitrate as the fused salt, to 480° C., when using silver chloride as the fused salt; and while it is not completely clear, it appears that the period for which the glass substrates were dipped into a fused salt bath and heated in a platinum crucible ranged from a period of from about 30 minutes to about 40 hours, depending upon the identity of the fused salt.
In Japanese Publication 04-338138, it is taught to produce an antibacterial glass powder by heating the glass powder to near its glass transition temperature and then dipping the heated glass powder into an ion exchange solution, e.g., an aqueous solution of a silver salt or copper salt, to exchange silver ions or copper ions for sodium ions initially present in the glass powder. This publication is discussed in Japanese publication 10-158037, where it is stated, in effect, that while the process might be capable of producing antimicrobial glass powder, it is unsuitable for producing antimicrobial glass sheets or plates because the process is incapable of producing sheets or plates having a uniform concentration of metal ions, e.g., silver ions or copper ions, in the front surface of the glass because of the surface tension of the ion exchange solution. A more fundamental reason why this process is not suitable for producing antimicrobial sheets or plates of glass is that the glass sheets or plates that are heated to near their glass transition temperature will explode upon insertion into the aqueous solution.
Other Japanese publications that relate generally to antimicrobial glass compositions include Japanese Publication 07-300339 and Japanese Publication 07-048142. Both of these publications, however, relate to the production of silver containing glass compositions wherein the silver component is introduced as part of the batch material that is used to form the glass. Moreover, the 07-300399 publication relates to antibacterial glass compositions that are rich in CaO, MgO and P2O5, rather than to more conventional soda lime glasses.
U.S. Pat. No. 4,507,392 relates to transparent glass-ceramics designed for application as glazes to low expansion ceramic bodies. The transparent glass-ceramics, which are not said to contain silver ions or any other antimicrobial metal ions, may be applied to the glass ceramic substrates by any means conventional in the art, such as by dipping, silk screening and spraying. Silk screening is said to be the preferred method of applying the glaze compositions. In that method, the glaze is applied as a paste comprised of glass-ceramic fritted glass, and optional pigments, in a non-aqueous vehicle consisting of a silk screening oil base and a volatile solvent. After screening the paste on the substrate, the paste is dried and then fired to fuse the particles to a smooth glaze or enamel. U.S. Pat. No. 4,440,810 is anther patent that relates to depositing glazes on glass substrates. In that patent, it is disclosed that a non-aqueous suspension of fritted enameling composition can be sprayed onto a ceramic substrate using fluid suspensions of the enamels in a conventional screening oil/solvent vehicle. Conventional screening oils include, for example, pine oil-based and boiled linseed oil-based screening oils or so-called squeegee oil compositions that are well known in the art. Specific screening oil bases that are disclosed in this patent include those which are commercially available from Ciba-Geigy Corporation, Plastics and Additives Division, Ardsley, N.Y. 10502, under the names Darkened 479, Darkened Medium 175, Darkened Medium 324, and Darkened™ screening medium. It will be appreciated that glazes formed by firing glass frits typically will not be transparent, that they are highly susceptible to mechanical damage, and that they would not be suitable for use in preparing antimicrobial food contacting surfaces, such as food cutting boards.
U.S. Pat. No. 6,197,366 discloses coating a glass substrate with a metal paste comprising an organo- or inorganometallic compound that is solid at ordinary temperature, and a viscous amino compound as a medium for the metallic compound. The paste, as a thick film, is applied to the glass substrate and is baked at a relatively low temperature (form 90° C. to 550° C.) to form a metal film in which the metal particles simply make contact. Among the metallic compounds that are disclosed for use in this patent are compounds of palladium, platinum, rhodium, gold, silver, copper, and others. The metal-coated glass substrates are said to be useful, for example, as an electrically conductive material, as a resistor material, as a heat insulating material, as a metal luster material, as a material for decoration or as a material for microbial growth-inhibition. There is no suggestion that transparent, colorless, sliver-coated glass substrates would be prepared by the disclosed process. In fact, examples that disclose the use of a silver compound in the paste formulation, also disclose that the resulting tin-coated glass is colored. See, for example, Examples 15 and 16 (silver-colored film), Example 17 (silver mirror film), Example 18 (slightly yellow-tinted silver film), Example 19 (gray silver film) and Example 20 (mirrored silver-palladium alloy film).
U.S. Pat. No. 5,085,416 relates to a sterilized cooking board comprised of a base board and an organic polymer layer containing an antibacterial zeolite. The polymer layer is formed on the entirety of at least one surface of the base board. The antibacterial zeolite is a zeolite in which a part or all of its exchangeable ions have been exchanged with antibacterial ions, such as silver ions, copper ions or zinc ions. There is no suggestion in this patent of forming the cooking board from an antimicrobial glass.
Published U.S. Patent Application 2002/00112760 relates to an antimicrobial food tray having a surface with which food comes into contact and which contains an inorganic antimicrobial agent, such as silver ions, copper ions, zinc ions or the like. The tray may be formed from a plastic resin that contains the antimicrobial metal ions, or the antimicrobial metal ions may be contained in a coating that is applied to the tray. There is no suggestion of forming the antimicrobial food tray from an antimicrobial glass.
While a number of processes are known for preparing antimicrobial substrates, there exits a need for improved processes that are capable of providing contact-killing, antimicrobial substrates that exhibit long lasting and safe efficacy. There also exists a need for a facile process for producing transparent, essentially colorless glass substrates that contain a contact-killing, non-leaching antimicrobial effective concentration of metal ions, particularly silver ions, in at least one surface region thereof.